Evidence for entropy effects in the reduction of ceria-zirconia solutions

摘要

An instrument for Coulometric-titration measurements was built and used to measure the thermodynamic redox properties for a 10 wt Cu/silica catalyst at 973 K and for two ceria-zirconia solid solutions, Ce0.81Zr0.19O2 and Ce0.25Zr0.75O2, between 873 and 1073 K. For Cu/silica, the equilibrium data show two well-defined steps in the oxygen isotherms, associated with equilibrium between Cu and Cu2O and between Cu2O and CuO; and the P(O-2) associated with these two steps are in close agreement with values expected for pure Cu. The oxidation enthalpies for both Ce0.81Zr0.19O2 and Ce0.25Zr0.75O2 were similar, between -500 and -550 kJ/mol of O-2, and independent of the extent of reduction. However, there is a step change in -Delta S of reduction for Ce0.81Zr0.19O2, from similar to 250 to < 100 J/mol center dot K, after removal of approximately one oxygen for every two Zr4+. A model is presented which views the reduction of ceria-zirconia as removal of oxygen from "pyrochlore-like" structures, with some of the changes in reducibility associated with the number of sites from which oxygen can be removed.